Printed electronics are increasingly finding uses in a great variety of applications, including portable electronics, signage, lighting, product identification, packaging flexible electronic devices (such as those that can be rolled or bent), photovoltaic devices, medical and diagnostic devices, antennas (including RFID antennas), displays, sensors, thin-film batteries, electrodes and myriad others. Printed electronics have a variety of advantages over electronics made using other methods, including subtractive methods. Printing can be faster than normal subtractive methods (such as etching) and can generate less waste and involve the use of fewer hazardous chemicals than in such methods. The resulting electronics can be more facilely used in flexible devices, such as displays, that are designed to be rolled, twisted, bent, or subjected to other distortions during use.
Printed electronics are typically made by printing the electronic circuit or other component or device on a substrate using an electrically conductive metal-based ink. The inks typically contain silver particles, and occasionally copper particles, other metallic particles, and/or conductive polymers. However, conductive polymers alone are generally not sufficiently electrically conductive. Furthermore, the resulting printed metallic circuits are usually insufficiently electrically conductive to be effective in most applications, including in devices in which the circuits are regularly stressed by bending and/or stretching during use. The printed substrates must therefore often be heated at elevated temperatures to sinter the conductive metal particles in order to achieve the desired levels of electrical conductivity. The temperatures used in sintering processes frequently limit the substrates that can be selected for the preparation of the electronics. For example, while it would be desirable to use inexpensive materials such as paper, polyolefins (e.g., polypropylene), and the like as substrates for printed electronics in many applications, the sintering temperatures often required are too high to be used with paper.
Furthermore, silver is costly and other, non-precious, metals can form oxides upon exposure to the environment that can render the material insufficiently conductive for the application. Additionally, the use of metal-based inks can add weight to the resulting device, and the aforementioned sintering process can add one or more additional steps, time, and complexity to the fabrication process. It would thus be desirable to obtain printed electronic devices using inks that do not contain costly precious metals, that are lighter weight, and that do not require sintering to become sufficiently electrically conductive, and that could therefore be used on a wider variety of substrate materials, including paper and polyolefins such as polyethylene.
U.S. Pat. No. 7,097,788 discloses a method of increasing the conductivity of an ink comprising orienting particles in the ink. U.S. Pat. No. 7,163,734 discloses an element and a method for patterning an organic polymer electroconductive layer that is suitable as an electronic circuitry element in an electric or semiconductor device. US 2006/0124922 discloses an electrically conductive ink used to form electrodes for an organic semiconductor transistor. WO 2004/006635 discloses a method of printing using an electrically conductive ink. WO 2006/108165 discloses a conductive ink containing fine metallic particles, a polymer base, a solvent, and a nanotubes containing conductive filler. WO 2006/137666 discloses an antenna having an antenna radiator formed by printing electrically conductive ink on a substrate. WO 2007/053621 discloses a method of electrohydrodynamic printing and manufacturing.